In applying voltages to voltage sensitive loads care must be taken that transients do not occur, especially when instrument power is being turned on or off. For example, when the output of a direct current power supply is to be varied in accordance with a computer program, the digital output of the computer is converted into an analogue voltage by a digital to analogue converter and applied, after amplification, so as to vary the reference voltage of the power supply. When power is turned on or off, the amplifier can provide transients of high amplitude that would cause the power supply to deliver dangerous voltages or currents to a load connected thereto. At the instant the instrumentation is turned on, it is also possible that the computer output or the output of the digital to analogue converter can have a value that would cause the voltage output of the power supply to damage the load which it is driving.